Seeking to bolster his conservative record ahead of the May 20 primary, the 30-second ad highlights votes Simpson has taken that tow the conservative line and labels his challenger, attorney Bryan Smith, as a "personal injury lawyer." The ad is backed by a $27,000 buy on broadcast and cable, according to a Republican source tracking buys in the district.

Smith's bid is backed by groups such as the Senate Conservatives Fund and the Club for Growth, whose political action arm launched a TV ad this week attacking Simpson's record as "just too liberal."

Whoever emerges from the primary will win this heavily Republican district, which includes much of Boise and the entire eastern portion of the state. GOP presidential nominee Mitt Romney, who endorsed Simpson's re-election bid , carried the district by a 31-point margin in 2012.

Idaho's 2nd District is rated a Safe Republican contest by Rothenberg Political Report/Roll Call.